Possible to connect two separate AC-coupled Quattro/PV inverter micro-grids over 1km apart

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serg-soton asked Oct 30, '19

Possible to connect two separate AC-coupled Quattro/PV inverter micro-grids over 1km apart

Two separate PV micro-grids (A and B) are at over 1km apart. We want to connect them so that they can support each other, particularly at night, when either A or B might shut down due to low battery. Assuming conductors can be specified that keep voltage drop within limits:

Would it be possible to connect the Quattros from AC OUT 2 (A/B) to AC IN 1 (B/A) as in first figure attached if (i) 'AC ignore' enabled on AC IN 1 (ii) no feeding in from AC IN 1 (A/B) back to AC OUT 2 (B/A) and (ii) no charging of battery from AC IN 1?

Or would a more feasible approach be to isolate one of the Quattros (e.g. A) from the AC loads using contactors and a PLC, to allow the other Quattro (B) to support the AC loads in micro-grid A? (second figure)

AC PV Coupling ac out 2 design

interconnect1.png (25.5 KiB)

interconnect2.png (34.6 KiB)

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Answer 1 · 2019-10-30T13:24:38Z

ejrossouw answered · Oct 30, 2019 at 01:24 PM

@serg_soton The voltage loss makes it non-starter, not to mention the high cost of running a suitable electrical cable. Since you provide no system information, I recommend you consider a battery upgrade (to lithium which has greater DOD, charges fast) and maybe other system components. I am not familiar with generator solutions, but that could also be a much better cost option to keep things going. Lithium however is my suggestion.

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serg-soton commented · Oct 30, 2019 at 04:43 PM

@ejrossouw Thank you for your suggestion and we will definitely consider a Li battery when upgrading the systems. However we have a project requirement to connect the two micro-grids. I am interested in whether this is possible in principle, rather than if it is the optimum economic solution. If the voltage drop was kept within the acceptable limits of the Quattro AC IN (maybe by step up/down to/from 11kV), are there other killer reasons why connection would not be possible?

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ejrossouw serg-soton commented · Oct 30, 2019 at 07:09 PM

I never came across such a requirement and of course it does not mean you cannot be the pioneer with a solution. I think this is where you need some transmission line experts involved which is out of my domain.

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Answer 2 · 2019-10-30T16:19:10Z

Phil Gavin answered · Oct 30, 2019 at 04:19 PM

Interesting problem, I don't want to dismiss it out of hand.

If you can't crunch the numbers the technical issues are moot, and that 1km connection won't be cheap. You may also have incompatible equipment, but maybe not, so I'll continue.

You'll need to cost out a step-up/step-down transformer combination, and probably have to construct a homemade overhead line. I don't think there is a cheaper solution for the 1km connection.

Then I think you may be over-thinking the problem:

I'd consider having both batteries and Quattro's at one end, the Panels and the PV inverter can remain at the remote end, so as to treat the remote end as a load ( and only having to use two conductors and transformers for the 1km). There will probably have to be an earth conductor run as well.

Is there a grid connection available at an end? -That's the end you put the batteries and Quattro's.

Two different battery types/Quattro's will be an issue as well, but I think now that problem is a lot more contained and possibly manageable.

You don't mention, but are these batteries/Quattro's compatible to be combined/paralleled?

It may be technically feasible, but probably not economically.

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serg-soton commented · Oct 30, 2019 at 05:34 PM

@Phil_Gavin Thank you for your response - food for thought. The two systems are identical but operated by independent entities, so need to stay physically separate. The Quattros are compatible but too far apart to communicate, as I understand it. There is no grid connection available at either end. Once upgraded, each system will consist of 3x10kVA Quattro + 2x15kVA Fronius grid-tie + 120kWh battery + 45KWp PV. They will be 3-phase systems (didn't mention previously to avoid too much detail in question).

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